1. Field of the Invention
This invention relates to methods of, and apparatus for eye surgery, and more particularly to a laser method and apparatus for corneal and intraocular surgery, as well as vision correction.
2. Description of the Related Art
With the development of laser procedures such as LASIK to help correct eye-sight or vision, by reshaping the cornea of the eye, several other new laser eye-procedures are being considered that concern the photo-ablation of eye tissue.
U.S. Pat. No. 4,538,608, issued to L'Esperance, Jr. for “Method and Apparatus for Removing Cataractous Lens Tissue by Laser Radiation” teaches how to deliver laser energy into the anterior of the eye lens and scan the laser beam in order to photo-ablate cataractous tissue, and has general importance for the process of photo-ablation of eye tissue, including photo-ablation of stroma. This procedure was improved by J. Bille (U.S. Pat. No. 5,246,435 “Method for Removing Cataractous Material”), who invented a procedure of laser energy delivery to separate lamellae in the stroma by focusing a laser beam between lamellae layers and photo-ablating tissue at the interface between these layers.
In those inventions, nanosecond (nsec) type laser beams were considered (for example, 10-20 nsec excimer lasers, or 5-10 nsec Nd/YAG lasers; 1 nsec=1×10−9 sec). With these pulse durations, each laser shot, in addition to ablating tissue, creates strong shock waves within the eye and generates significant tissue heating. These effects are undesirable, and may be reduced by using lasers with shorter pulse durations. Therefore, when compact ultra-short lasers, those with pulse durations less than 1 picosecond (1 psec), [1 psec=10−12 sec], were developed in the late 1980's, they were considered for use in eye surgery.
In the review article by Christopher Yo et al. on “LASIK, Future Advances” (E-Medicine, Nov. 25, 2004) the authors stressed (page 5) that “ . . . one can assume the culprit that negates all the advantages of custom ablation may lie in the flap procedure itself. Hence, it would be a great leap in refractive surgery if the LASIK procedure can one day be completed intrastromally without the need for cutting a flap.” In addition, the LASIK flap may lead to complications such as flap striae, epithelial ingrowths beneath the flap, diffuse lamellar keratitis, and flap tears. The present invention responds exactly to the desirable outcome of corneal refractive surgery without a flap, namely reshaping the cornea by means of using high intensity femtosecond (fsec) laser pulses for correcting the refractive errors of myopia, hyperopia, and astigmatism without cutting a flap (1 femtosecond=1 fsec=1×10−15 sec).
The general advantage of using fsec lasers for eye surgery compared to using much longer pulse lasers (nsec-type excimer, Nd/YAG or Nd/Glass lasers) is that with fsec lasers there is a much lower energy requirement, in particular when the surgery requires eye tissue ablation, that is, photo-ablation. Photo-ablation is a thermal process that requires a certain intensity of laser beam, typically in the range of 109-1011 W/cm2. For the same ablated spot size, the intensity required for photo-ablation is inversely proportional to the pulse duration. For example, laser pulses of 100 fsec duration can provide photo-ablation at hundreds of times smaller beam energies than when laser pulses of 10 nsec duration are used. Being able to use smaller beam energies, ultra-short laser pulses can provide tissue cuts with less eye trauma, as was proven experimentally. This observation leads to three principal advantages of using ultra-short laser pulses for eye surgery. One advantage is that it is possible to perform much higher precision tissue cuts with such lasers when compared with nanosecond-type lasers. A second advantage is that ultra-short laser pulses produce much smaller heating effects in tissue when compared with longer laser pulses, greatly reducing tissue damage. A third advantage is that ultra-short laser pulses produce only very weak shock waves in tissue, whereas long laser pulses produce very substantial shock waves resulting in considerable trauma. In eye surgery, this trauma can have substantial negative effects on the prognosis following surgery, such as inflammation and undesirable wound healing.
In addition to photo-ablation, laser pulses can be used to produce photo-disruption, which is also a thermal process. The photo-disruption process can result in the formation of bubbles, i.e. cavity bubbles or gas bubbles in the tissue. This requires significantly less intensity than photo-ablation, typically in the range of 108-109 W/cm2.
In conventional LASIK procedures, where a flap is created by using a mechanical microkeratome, photo-disruption provides the basis for replacing the mechanical flap cut with a much more precise flap cut using a fsec laser [Juhasz et al., U.S. Pat. No. 5,993,438 (issued Nov. 30, 1999) “Intrastromal photorefractive keratectomy”, T. Juhasz, U.S. Pat. No. 6,110,116 (issued Aug. 29, 2000) “Method for corneal laser surgery”, and T. Juhasz et al., U.S. Pat. No. 6,146,375 (issued Nov. 14, 2000) “Device and method for internal surface sclerostomy”]. U.S. Pat. No. 6,146,375 also teaches about using fsec or picosecond (psec) pulses for the treatment of glaucoma. T. Juhasz et al.'s research has led to the successful company “IntraLase” that markets the procedure for cutting the flap with a fsec laser in preparation for LASIK eye surgery, where the corneal correction itself uses an excimer laser providing pulses of 10-20 nsec duration.
In The patent application “Method and Device for Corneal Reshaping by Intrastromal Tissue Removal” by S. Suckewer, P. Hersh, A. Smits, and A. Morozov, published by US Patent Office on Feb. 28, 2008 (Pub. No: US 2008/0051772 A1) a new approach to cornea reshaping is described. This invention teaches how to apply a laser beam to reshape the eye's cornea under the cornea's surface without creating or removing a flap, hence it is called Flapless LASIK.
Flapless LASIK is a two-step procedure. The first step creates long and narrow channels in the cornea with a laser beam oriented approximately perpendicular to the axis of vision. Such channels lead to the regions that are to be ablated. In the second step, the fsec laser beam, through such channels, reaches the stroma between the inner (endothelial) and outer (superficial) cornea and results in ablation of the stroma at a spot. By changing the position of the focusing lens or focusing mirror, the location of the ablation spot moves along the channel. Controlling the number of laser pulses for each spot controls the amount of ablated material in each spot. The preferred laser beams consist of ultra-short pulses of duration 30-200 fsec, although the duration could be shorter or longer, at repetition rates of 1,000-10,000 Hz, with higher or lower repetition rates possible as well, and intensity in the range of 1013-1015 watts/cm2, resulting in what the inventors termed “multi-photon ablation”.